The present invention relates to a method for the production of a structured metal layer made from an alloy composed of titanium and nickel.
Structured metal layers or metal foils produced in such a manner may be used in particular as a biocompatible implant, for example as embolic filters or as bands and generally as joining elements between the bones of the human skeleton. With appropriate shaping, such layers may also be used as stents in blood vessels. Thus the invention further relates to an object, particularly a stent or an implant, which comprises at least one layer produced according to this method.
The alloy with titanium and nickel used for the production of the metal layer advantageously has superelastic behaviour and/or shape memory properties. Materials with shape memory properties (SM materials) are characterized in particular in that they can be deformed in a low-temperature phase with martensite structure and after subsequent heating in a high-temperature phase with austenite structure they remember this impressed shape and return to it. A frequently used property of such materials is their superelastic behaviour. Within a specific time interval above a characteristic preload, which may be several hundred MPa, a plateau arises in the stress-strain curve. The austenite transforms into martensite within this strain range. The stress-induced martensite may decouple according to the stress applied and thus facilitates deformation of the material within the plateau under constant counterforce. At the same time, expansions up to approx. 8% may be introduced into the stress-induced martensite by way of the phase transformation without plastic deformation occurring. On relieving the load on the martensite, it transforms back to the initial condition of the austenite with a hysteresis with respect to the plateau stress.
Due to their good biocompatibility, materials made from nickel-titanium alloys (NiTi) are frequently used in medical technology. The superelastic properties of the nickel-titanium alloys are advantageous in medical tools such as catheters, which are used, for example, for stent positioning and which are exposed to severe deformations during their use in the body. Tissue spreaders with superelastic properties have the advantage that they cause less damage to the tissue than spreaders made from other materials. The shape memory effect may additionally be utilised in implants such as stents or embolic filters. In this case the implants are deformed in the martensitic state at room temperature. Subsequently, the deformed implants are inserted into the body where the high-temperature phase austenite is stable at body temperature. In the process, the implant transforms and remembers its original shape. The folded stents and embolic filters can thus unfold of their own accord.
Basically, the proportion of nickel in the alloy used for production of the metal layer may be varied within large limits between 2 and 98 atomic % depending on the application case. Preferably, however, it is suggested that the nickel content in the alloy should be between 45 and 60 atomic %.
Customarily, in implants, a compact material that is produced using conventional manufacturing techniques is used. It has, however, emerged that porous forms of the compact material are advantageous in medical implants since cells, particularly stem cells or bone cells, may in the process grow into the pores, thus ensuring better embedding of the implant in the bone or into the new tissue to be created. Thus, where stents and embolic filters are concerned, mesh-like structured shapes are also used. Typically, shaping of the stents is performed by means of a combination of deep drilling and laser cutting from solid material.
Furthermore, it is known to produce thin shape memory layers with a superelastic behaviour by means of physical deposition methods, especially by means of sputtering. At the same time, in addition to the advantage of using less material, sputtered layers of nickel-titanium alloys primarily enable the production of structures with smaller dimensions.
It is known, for the introduction of a structure into a layer of a nickel-titanium alloy sputtered as a closed surface, to subject the nickel-titanium layer to an etching process whereby photolithographic methods in particular may also be used. Lithographic structuring of nickel-titanium alloys is performed by means of a lithography step in which the desired structural shape is applied to a lacquer by means of masking. The light-sensitive lacquer is subsequently exposed and developed. The lacquer which is left behind in the process protects the regions of the structure, which should be retained, during further etching steps.
Nickel-titanium alloys may be etched wet-chemically using a hydrofluoric acid mixture (HF). The wet-chemical process is isotropic with the result that an undercut is also always produced in the process and the edge structure is adversely affected. In addition, undercutting, the level of which increases with thicker layers, also leads to the structural resolution being destroyed or the width of the ligands being lost. A further disadvantage exists in the etched surface remaining, which contains obvious signs of etching, with the result that if necessary a further surface treatment in the form of repolishing is needed. Furthermore, hydrogen may be introduced into the nickel-titanium alloys during chemical etching which may impair the shape memory properties or the superelastic properties.
Dry etching is another etching process in which the material to be removed from the nickel-titanium alloy is eroded or “sputtered” away by using an argon ion beam. This method is anisotropic with the result that there is no undercutting and the edge structures may be produced very cleanly. The method also prevents the introduction of hydrogen. One disadvantage of this method, however, is the low etching or erosion rate of just a few tenths of a nm/s which means considerable time expenditure. With layer thicknesses greater than 2 μm there is also the problem of redeposition where the material already eroded re-adheres to the surface.
In addition to these methods, by which the structure is not introduced into the metal layer until deposition has taken place, it is also known to introduce a structure into the substrate material onto which the nickel-titanium alloy layer will subsequently be deposited and which thus exists immediately in the desired structure.
Thus, using a method of the type referred to at the outset, which is known from International Patent Application Publication No. WO 00/04204 or from the corresponding German specification translation DE 699 20 712 T2, firstly a sacrificial layer composite is provided which comprises at least two sacrificial layers applied one on top of the other. One of these two sacrificial layers is subjected to a wet-chemical etching process for the provision of a structure during which undercutting of this sacrificial layer takes place. Subsequently, a metal layer of the alloy is applied indirectly or directly to the sacrificial layer composite already structured.
With this known method, the isotropic etching process gives rise to the disadvantages referred to above of signs of etching in the surface of the sacrificial layer which is etched wet-chemically. An especially smooth surface of the substrate or of a sacrificial layer used for deposition is, however, of major significance in achieving a high breaking strength of a sputtered nickel-titanium alloy. If crack nuclei in the shape of notches or pores are generated during layer production, then a material failure occurs in the tensile test at much lower stresses than the theoretical breaking strength. Local stress peaks, which exceed the breaking strength limit, are then reached in the material. Such stress peaks arise in notches, as represented by pores in the interior and scratches on the surface, due to a stress concentration.